The present invention relates to arrangements for controlling the pressure of hydraulic braking fluid supplied to wheel brake actuating cylinder incorporated in a vehicle braking system in general, and more particularly to control arrangements of this type which can be used in vehicle braking systems provided with antiskid control equipment.
There are already known various constructions of arrangements for controlling the pressure supplied to the wheel brake actuating cylinders incorporated in vehicle braking systems, among them such which are particularly or exclusively suited for use in vehicle braking systems provided with antiskid control equipment and with an auxiliary source of pressurized hydraulic fluid, as well as with a brake pedal actuated master cylinder device. It is also known to provide the master cylinder device, which includes at least one master piston circumferentially sealed by a lip seal, with at least one associated replenishment chamber and with a feeding port, and to arrange a throttling valve device which is incorporated in a circulating circuit between the master piston and the brake pedal. The antiskid hydraulic braking system usually includes electromagnetically operatable control valves in the supply conduit between the working chamber of the master cylinder device and the associated wheel brake actuating cylinders. The hydraulic fluid discharged from the respective wheel brake actuating cylinders during the antiskid control operation flows through a return conduit to a low-pressure supply reservoir. In conventional constructions of this type, there is further provided an auxiliary energy source including a hydraulic pump drawing fluid from the low-pressure supply reservoir, there being provided a hydraulic connection between the inlet of the circulating conduit to the throttling valve device and the replenishment chamber associated with the master piston.
In one conventional construction of this type, which has been disclosed in the published German application DE-OS 24 43 545, the circulating conduit, which leads from the low-pressure supply reservoir through the hydraulic pump to the replenishment chamber associated with the master piston and from there to the secondary side of the master piston, includes a longitudinal bore in the piston rod of the master piston and leads from this longitudinal bore to the throttling valve device having a radial outlet which is connected with the supply reservoir so as to close the circuit. The throttling valve device includes an inner throttling valve member which cooperates with associate portions of the free end of the piston rod of the master piston to control the flow of the hydraulic fluid in the circulating circuit. A compression spring which is arranged between the piston rod of the master piston and the throttling valve member holds the end of the piston rod of the master piston which is closer to the brake pedal at a distance from the throttling valve member, so that flow through the throttling valve device is unobstructed and the continuously operated hydraulic pump causes the hydraulic fluid to idly circulate in the circulating circuit, so long as the brake pedal is not depressed. On the other hand, once the brake pedal is depressed, the throttling valve member engages the corresponding regions of the free end of the piston rod of the master piston, so that the throttling valve device closes and interrupts the flow of the hydraulic fluid in the circulating circuit, thus terminating the idle mode of operation of the hydraulic pump. As a result of the closing of the throttling valve device, the pressure at the secondary side of the master piston rises and causes the continuously delivered hydraulic fluid to flow from the secondary side of the master piston through the supply bores provided in the latter into the working compartment of the master cylinder device, from where it flows through the supply conduits into the wheel brake actuating cylinders while dynamically controlling the operation thereof. The master piston of the master cylinder device remains stationary during such operation. The normal function of the master cylinder device with a static control of the operation of the wheel brake actuating cylinders occurs only when the auxiliary energy supply system fails and, consequently, the operation of the wheel brake actuating cylinders cannot be dyanmically controlled any longer.
This conventional construction is possessed of several important disadvantages. So, for instance, this conventional construction consumes a relatively high amount of energy because of the continuous operation of the hydraulic pump. Another drawback is that it is impossible to test the sealing effect of the annular lip seal of the master piston in the direction from the working compartment to the secondary side of the master piston even during the normal operation of the braking circuit without antiskid control action. This may have very grave consequences, since the annular lip seal may become defective first but the existence of the defect is not recognized so long as the auxiliary energy source is operational so that, should the auxiliary energy source subsequently become inoperative, it is impossible to perform emergency braking operation with static control of the pressure supplied to the wheel brake actuating cylinders, due to the defect of the annular lip seal of the master piston and the resulting impossibility to build up static pressure in the working compartment of the master cylinder device.